The invention is based on a priority application EP 01440356.2 which is hereby incorporated by reference.
This invention relates to a circuit arrangement for controlling the electric power to be supplied from a direct-current source to a load, particularly to a fan or heater in a motor vehicle.
To control the electric power to be supplied from a direct-current source to a load, use is frequently made of power field-effect transistors which are simply connected in series with the load to be controlled. Their use is relatively unproblematic; in particular, they are simple to control.
A frequent field of application is that of fans or heaters in motor vehicles. These operate on a voltage of 12 volts from the on-board battery. Because of the low voltage, relatively high currents occur. For instance, a load with a power of 360 W requires a current of 30 A. During the control of this power, a maximum power dissipation of 90 W occurs in the controlling power field-effect transistor. Suitable field-effect transistors are at least very expensive. Because of the high temperatures caused by the high power dissipation, their life expectancy is generally short.
In principle, field-effect transistors can also be used to turn loads on and off under clock-pulse control and thus achieve power control. In that case, relatively little power is dissipated. Particularly because of the existing tendency towards radio interference, which is complicated and costly to get control of, this solution is frequently not desired. By connecting field-effect transistors in parallel, it is possible to use less expensive transistors, but the thermal problems are reduced only slightly. In addition, the currents do not readily divide in equal shares.
The object of the invention is to provide a circuit arrangement of the above kind which permits a low-cost solution to the problem.
According to the invention, this object is attained by a circuit arrangement for controlling the electric power to be supplied from a direct-current source to a load, particularly to a fan or heater in a motor vehicle, the circuit arrangement comprising a field-effect transistor to be connected in series with the load to be controlled and a control circuit for controlling the field-effect transistor, the field-effect transistor having a second field-effect transistor connected in series therewith, a resistor being connected in parallel with one of the field-effect transistors, and the control circuit being adapted to control the two field-effect transistors in the sense of a fadeover between two final states in each of which one of the field-effect transistors is off and the other is on.
The invention is based on the idea to largely shift the inevitable power dissipation in a low-cost analog solution to a low-cost resistor. In this manner and with a suitable circuit design, even at the most unfavorable operating point, the two field-effect transistors used in accordance with the invention together have to dissipate only half as much power as if only one such transistor is used. Further developments of the invention are apparent from the subclaims and the following description.